Bremsstrahlung induced nuclear reactions with Eγmax = 450 MeV

I 2.1 [ Nuclear Physics A221 (1974) 145--162; (~) North-HollandPublishiny Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

B R E M S S T R A H L U N G INDUCED NUCLEAR REACTIONS ITIS tX W I T H E~ = 450 MeV

P. DAVID, J. DEBRUS, F. L~BKE, H. MOMMSEN, R. SCHOENMACKERS and G. STEIN lnstitut fiir StraMen- und Kernphysik der Universitiit Bonn, D 53 Bonn, Germany

Received 12 October t973 (Revised 14 December 1973)

Abstract: The target nuclei 27A1, 4°"*'tEa, 5IV, 59Co, 93Nb, n'~Ag, lSXTa and 197Au were irradiated with bremsstrahlung ofEy "~ -- 450 MeV. Protons, tritons, 3He and 4He particles were detected on-line with an identification system using surface barrier detectors. Energy spectra, yields and angular distributions are given. The energy spectra are compared with calculated spectra from the compound nucleus model.

E [ NUCLEAR REACTIONS 27A1, 4"°'4'*Ca, slV, sgco, 93Nb, "atAg, lSlTa, 197Au

I (7; p,t, aHe, 4He), E.. ' ~ * : 450 MeV, measured aq (Ep~tt~) at 90 ° , yields.

1. Introduction

It is well known from the forward-backward ratios of fission fragment activities 1, 2) and f rom the anisotropies of the angular distributions of light ions 3) (A = 2-14) originating in irradiation of heavy nuclei with high energy particles (E > 1 GeV) that an intermediate nucleus with low momentum but with high excitation energy is formed. From the comparison 4) of photon induced fission and proton induced fission of nuclei with A > 150, conclusions can be drawn about the absorption mechanism of 7-quanta in the energy range from 50 to 450 MeV [refs. 2, 5)]. In the range 50 to ~ 250 MeV mostly absorption on correlated nucleons takes place. Above 250 MeV the production of pions and reabsorption by correlated nucleons become important. The pions are again predominantly absorbed by quasideuterons and about the same energy is transferred to the nucleus as in the lower energy range. These results show that not only is the direct excitation of the nucleus important but also the excitation of the nucleons and the following secondary excitation of the nucleus by the particles emitted from the decaying nucleons. These processes become important in reactions induced by ?-rays with energies above ~ 180 MeV [ref. 6)] and in reactions induced by nucleons and heavy ions with energies above ~ 1 GeV per nucleon 7).

In this paper we report on the measurement of energy spectra of light charged particles (p, t, 3He, 4I-Ie) emitted from target nuclei with A = 27 to 197. The targets were irradiated with bremsstrahlung of E~ ax = 450 MeV. Photon induced reactions

145

146 P. DAVID et aL

have cross sections ranging from millibarns in the giant dipole resonance (GDR) region to microbarns at higher energies. Therefore, on-line measurements such as we have performed have the great advantage of yielding simultaneously direct in-

formation with the same normalization of different reaction channels with low cross sections. Since bremsstrahlung has a continuous intensity spectrum, all excitation

energies will occur, the excitation of the target nuclei depending on the y-absorption process by which energy is transferred to the nucleus. With the information from the forward-backward ratios of the formation of intermediate nuclei in near equilibrium states we expected the particle spectra to be continuous and of evaporation type. The

final spectra will be a superposition of spectra resulting from the decay of nuclei with various excitation energies. Therefore an attempt is made to interpret the measured spectra with the evaporation model. Detailed Monte Carlo calculations such as reported in ref. s) might give further insight into the T and pion absorption

processes by correlated nucleons and the resulting excitation and following emission of particles.

In sect. 2 the experimental set-up is described, and the experimental results are summarized in sect. 3. The statistical model applied to interpret the spectra is out- lined in sect. 4 and the discussion of the results is given in sect. 5.

2. Experimental 2.1. THE BEAM

The measurements were carried out at the Bonn 0.5 GeV electron synchrotron. The maximum energy of the bremsstrahlung was E~ "x = 450 MeV. The details of the machine are described elsewhere 9).

After leaving the vacuum tube of the accelerator, the ~,-beam was confined by two lead collimators of 3 x 3 mm 2 and 5 × 5 mm 2 cross section and cleaned in a sweeping

magnet. The beam hit the target at an angle of 45 ° in a vacuum chamber, 25 cm in

diameter. The target distance from the tungsten target was 450 cm. The chamber was evacuated to less than 10-2 Torr. The beam spot on the target formed an area of

25 mm 2.

The position of the beam on the target was checked prior to and after each measurement. The total beam energy was measured in a Wilson-type quantameter located

R K1 K2 RM S

L . ~, k L.___~, k I a ~ _ '

Fig. 1. Beam handling at the 500 MeV electron synchrotron. R - synchrotron ring; KI, K2 - collimators; RM - sweeping magnet; S - scattering chamber; Q - quantameter; T - target;

Z -- detectors; a = 100 cm, b = 20 cm, c = 100 cra, d = 170 cm, e = 1060 era.

BREMSSTRAHLUNG INDUCED REACTIONS 147

,,~ 15 m behind the tungsten target. A schematic outline of the beam handling to the target place is shown in fig. 1.

2.2. THE DETECTION SYSTEM

A particle identification system of the Goulding type was used to separate protons, tritons, 3He and 4He particles. The telescope consisted of two Si(Li) semiconductor surface barrier detectors, determining the mass, charge and energy of the particles. A third Si(Li)detector was connected in anticoincidence with the other two detectors, in order to reject pulses of particles which were not totally stopped by the first two detectors. The telescope was mounted in the scattering chamber 4.5 cm from the target at an angle of 90 ° to the beam direction. The thickness of the detectors combined in different measurements and the corresponding energy ranges are listed in table 1.

TABLE 1

Thickness of detectors and corresponding energy range of the particles from the different targets

Target Detector thickness Measured energy range (Itm) (MeV)

d E E ~cj IH 3H 3He 4He

27AI S9Co 61.5 2000 2000 .atAg

197All

4°Ca 37.1 2000 300 44Ca 99 2000 300

sl V 93Nb 37.1 1000 500 8 i Ta

3 -9 4.5-20 8.5-40 9.5-40

7 -32 7.5-34 3.5-9 5.5-20 12 -44 13 -50

7 -25 7.5-25

In order to minimize the high background, the intensity of the electron beam had to be kept sufficiently low. A permanent magnet of 500 G was mounted in front of the AE detector to deflect low energy electrons from the target.

A 1 mm thick brass housing put around the detectors reduced the background scattered back from the walls of the chamber. The background measured with an empty target holder amounted to about 20~ for the proton and triton spectra, whereas no background was measured in the 3He and 4He spectra.

A particle spectrum measured in the irradiation of a thick Be target is shown in fig. 2.

The energy resolution of the energy spectra was better than 200 keV.

2.3. THE TARGETS

The characteristics of the irradiated targets are listed in table 2. The purity of the targets was greater than 99.9~. The target thickness was determined to be within 10~ of the listed value.

148 P. DAVID et al.

200

180

160

27 140 < "i" C~

120

(21 ~100-

3 H

9B e . y

E ~ x = 450 MeV

80-

60-

t,0-

, L~ - . . . , . . . . ~ ,~ - : , . ~ . ~ . , ~ . -~ ,~ - . a~ r ~ . . . . . . . . . . . . . . . . . . . . . " r . ' : ", ,

100 200 300 400 500 600 700 800 900 1000

C H A N N E L

Fig. 2. Mass spectrum of particle identifier output for the reaction 9Be i-7, Erm"" - 450 MeV.

TABLE 2 Thickness ofthe targets

Target Thickness Target Thickness (mg/cm 2) (mg/cm 2)

27AI 2.4 -t-0.24 93Nb 3.94-0.4 4°Ca 0.854-0.09 "atAg 8.2&0.8 '*'*Ca 0.90--0.09 t s ~Ta 2.8 4-0.3 5iV 2.7 --'0.3 t97Au 1.9±0.2 59Co 0.80-.1-0.08

3. Experimental results 3.1. THE CROSS SECTIONS

From all the energy spectra of protons, tri tons, 3He and 4He particles measured in the i r radiat ion of the target nuclei 27A1, 40, 44Ca ' 5iV, S9Co ' 93Nb ' .atmg '

~StTa and 197Au a characteristic choice is displayed in figs. 3 and 4. The energy of

the particles was corrected for the energy loss in the target material and for the

recoil energy of the residual nucleus,

Trec = (1 + m p / m r ) T t ,

B R E M S S T R A H L U N G I N D U C E D R E A C T I O N S 149

20,

0

I 2O I00

5O

i

0 15 25 5 15 25

et C,,v] e, CM.v]

Ag (y,p)

C

,,

5 ~0 Ep [MeV]

o~ ° s ~s ~ s 3s e ~ [ ~ ]

Fig. 3. Characterist ic measured spectra o f p, t, aHe and +He f rom the target nuclei 27AI, +o, ++Ca, natAg , 197Au .

where mp is the mass of the particle, m, is the mass of the residual nucleus, and Tt is the particle energy after target thickness correction. The cross sections are given per equivalent quantum number, per steradian and per kinetic energy interval 0.5 MeV of the emitted particle.

3.2. T H E E R R O R O F T H E C R OS S S E C T I O N S

The total experimental error of the cross sections consists of the error of the target thickness ( ~ 10~o), the error of the quantameter constant (..~ 5~o), the error in the determination of the solid angle ( ~ 5~), the error generated by the background subtraction and the statistical error. The latter is large in the 3He and 4He spectra as a result of the very low counting rate. This leads to a total error in the maxima of the x-spectra of about 30% and in the maxima of the proton and triton spectra of about 20~. The error in the cross sections of the aHe particles is 100%.

150 P. DAVID e t al.

Y

[ JS / l~ ~ O} 13 0 QH J

++ +-" o o

p. ~ 9

qd J o

!

c ~

1

4 h--..j

r~

l i i + ,i i i i i

o [jsA.k~ + 0 ] bO + L q d J

< g

E<

~ P

c -

o O

e~

++ m E

c~

++

c. - - I _ r

t_-~

C~r-+

z +

[±s^•; o I b till • O

o m:

i.F_,


102

101

I: ~ I0 0

i0-I

/ \

. . . . ~

J

D { y , ' ~ (~.' BARgiER)

• ( Y , p ) E#=3.5-8.5 MeV

A ( y , t ) E t = 5-20 MeV • (V,c[} Ea= 10-42 MeV

• {y lkll~ E3Ht=19-30 MeV

I 150 : 5~0 "09 200 A

Fig. 5. Yield of protons, tritons, 3He and 4He depending on mass number A[ref. 23)]. The lines through the points are to guide the eye.

To compare the particle yields of the different target nuclei the cross sections are integrated over kinetic energy intervals equal for each particle. An energy interval o f 5 MeV ranging from 3.5 to 8.5 MeV was chosen for the proton spectra, a 10 MeV interval for the triton, 3He and 4He spectra, ranging from 5 to 15 MeV for the tritons and from 10 to 20 MeV for both 3He and 4He particle spectra. Fig. 5 shows these integrated cross sections in mb/sr.

4. The statistical model

Because of the similarity of the shape of the energy spectra with the neutron induced particle spectra it seemed useful to calculate the energy spectra in the statistical model. Despite the expected crudeness of this model in this case of high excitation energies, the calculations were performed to find out the range of validity of several assumptions. The model excludes most of the possible reaction channels,

152 P. DAVID et aL

e.g. multiple particle emission, fission, deformation in the transition state, when particle and residual nucleus become separated, and particle emission from a preequilibrium state.

In terms of the energy distribution of the incident bremsstrahlung, the cross section for particle emission per equivalent quantum per energy interval of the particle, dT, and per solid angle is given in the statistical model by Weisskopf lo):

E ' / m a x _ _ _

n(T)dT = CTdTa~p.rQ a~,~ p(E;., T)N(E7 ~, E~)dE.~ (b/s 0, (1) ,J La +,a Fro t

where: T is the kinetic energy of the emitted particle; C is a well determined constant; crcp~rt(a~r ) is the cross section for formation of the compound nucleus, if particles (7-quanta) are used in the entrance channel; Fro t is the total decay width of the compound nucleus; p(E,e, T) is the level density of the residual nucleus, depending on the excitation energy U = E~.-EB-T, E B being binding energy; N(E~ ~, E.~) is the intensity spectrum of the bremsstrahlung ( ~ Q/E); A is the pairing energy; and Q is the number of equivalent quanta.

In the calculation the following approximations were applied: (i) The cross section a~ o~t is approximated by the continuum cross section given

in ref. lo). This approximation is good when using a high incident energy. (ii) We set act = at, t o t " The total 7-absorption cross sections in the low energy

region (E~ < 20 MeV) are taken from ref. 11). In the higher energy regions above 30 MeV the quasideuteron and pion absorption cross sections are calculated as

follows: trqa ~ 12 [(NZ)/A]a d [from ref. 12)];

ad = 70 pb,

a d = 6/E 2 ltb,

a~ = 330 A/~b,

E~ = 50-300 MeV [ref. 13)];

Er > 0.3 GeV [ref. 14)1;

Er > 150 MeV [re/'. 15)1.

The values between 20 and 50 MeV are linearly interpolated. (iii) We set/-to t = Fp + F , + F, and all other possible particle decay channels are

neglected: ~ Tlmax

r , = GJEB,+~T,p(E ~, T,)~o,(~)dT~ (i = n, p, ~), (2)

where EB, is the binding energy, A t is the pairing energy and T is the kinetic energy. If all three channels are open, the F, width is the main part of F~o t (approximately two orders of magnitude larger than Fp or F~).

(iv) The level density p(ET, T) was calculated by a constant temperature approximation as given in ref. 16) with the level density parameter a = A/7.9 and the pairing energy Ace = 12/x/A for doubly even nuclei, A,o = Aoo = J2-Aeo for even-odd and odd-even nuclei and Aoo = 0 for doubly odd nuclei.


In this model the cross section per equivalent quantum and per particle energy interval depends mainly on the level density parameter of the residual nuclei in the different exit channels and on o'er, the compound nucleus formation cross section. The calculations show that only the ratios of the various level densities occuring in the formula are of importance. Variation of the level density parameters with a fixed ratio of the parameters does not produce a large change of the calculated spectra. Application of Z 2 searches to the 7-spectra, the ratio of level densities being the only free parameter, showed that for all residual nuclei the assumption a = A/7.9 gave reasonable results.

(v) This approximation concerns the cross section for the formation of a compound nucleus by photons. If a low energy photon is absorbed, E 7 < 50 MeV, the cross section is assumed to b~ equal to the total ?-absorption cross section. For higher energies E = 50--450 MeV two interaction modes are assumed: first, that no formation of the compound nucleus occurs and no par~cles are evaporated; second, that the compound nucleus is formed with a cross section equal to the photon absorption cross section in this energy range. The real cross section will be between both modes.

10

~>~

10

10

100

10-1

27AI .¥,pl

o o

o

o o

° °

"f \ \ N

o 40Ca~y, p ] o

o

o e

° o o

/ X I \

/ ',

I \ I

I I

5 10 5 10 Ep [Mw]

Fig. 6. Experimental data with statistical model calculations for the reactions 27AI(~,, p) and 4°Ca(~,, p). Broken line for Er m~ = 50 MeV (calculation). Full line for Er m~x = 450 MeV (fit).

N

jz//

-°

i/I

,

g Z

s /

I |

--

I

~b

1

1

~ ~

o ~

°° [

~-~

]

°;"

, ~

s i

S

6 i

v1

> ,<

e~


With these assumptions the 4He particle and proton spectra were calculated. The final formula is:

n( Tb)d Tb = Tb d Tb acb( 2Sb + 1)mb

× f E y m ~ , ac'~(ET)p(U)N(E'~'~r .... E~)dEe , (3)

Z b'B'(2Sb, + 1)mb,j 0 b Tb, trcb,(Tb,)p(U')d Tb, ,/ 0

where a.~(a¢b, ) is the cross section for formation of a compound nucleus by incoming particle b(b ' ) ; Sb and r% (S b, and mb, ) are the spin and mass (MeV/c 2) of outgoing particle b(b ' ) ; N(E'~ ax, Er) .~ Q/E~, Q being the number of equivalent quanta; p(U) is the level density of the residual nucleus; B = t a rg e t -b ; U is the excitation energy of the compound nucleus; and U' is the exitation energy of the residual nucleus.

The results of these calculations are shown together with the experimental 4He and proton spectra of the target nuclei 27AI, 4°Ca, 93Nb and 197Au in figs. 6 and 7.

5. Discussion

On the basis of the statistical model with the assumptions just described, absolute cross sections were calculated for the 4He and proton spectra.

5.1. THE (y, ~) REACTIONS

Some of the calculated spectra are compared in fig. 7 with the experimental data for the target nuclei 4°Ca, 93Nb and 197Au. The calculations, based on the

assumption that no compound nuclei are formed for E~ > 50 MeV, show the following trends: the cross sections rise to a maximum value and at higher particle energies fall off nearly exponentially as expected in an evaporation type reaction. The position of the maximum is outside the measured energy interval for the spectra of the light nuclei 27A1, 40, 44.Ca ' 51V and 59Co. With increasing mass number A the

position of the maximum of the spectra shifts to higher energies of the ~t-particles. This is in agreement with the experimental spectra (table 3). This shift is a direct

TABLE 3

Experimental and calculated position of the maximum of the (7, g) spectra for E7 ma" = 450 MeV

Nucleus 27A1 4°C a 5IV 99C O 93Nb n~tAg 181Ta X97Au

Experimental position 5.5 ~) 7.7 a) 11.5 12.0 18.0 19.5 (MeV)

Calculated position (MeV) 7.0 8.0 9.0 11 12.5 17.5 18.5 ETmax = 50 MeV

The error is ~ 10~. 2) Rcf. 17).

1 5 6 P . D A V I D et al.

E

CM,v]

2O

/ / "

1 1 f l /

/ . i "

/ / . i

• < I / / / / /

/

x ~ elol . • l<s-egqar et ol. • I d e r ~ t l l , e oL • Erda, et aL * Ko~'n(:tr et tal. • I-~ffi~r~r~ el GI. o Wecldling eCal

= this ~tork

I I I I I 10 20 30 t,O 50 60

I I 70 80 Z

Fig. 8. The position of maxima of the (~,, ct) spectra and the width of the spectra at half height. The broken line gives the height of the Coulomb barrier.

consequence of the A-dependence of the c~-particle penetrability, which in the calculations is contained in the factor Tacva,, reflecting the height of the Coulomb barrier.

In fig. 8 the position of the maximum and the width of the z-particle spectra are displayed together with the height of the Coulomb barrier; values from the literature 17) are included.

The evaporation model based on the assumption that no compound nuclei are formed by )'-quanta with Er = 50-450 MeV gives too low cross sections for the ()', ~) reactions (fig. 7) for c~-particle energies above the position of the maximum. Taking into account the compound nucleus formation by the high energy part of the bremsstrahlung (E~ > 50 MeV) the tail of the calculated spectra is raised, the position of the maximum and the absolute value of the maximum cross section remaining almost unchanged with nuclei of A < 40. For nuclei with A > 40, however, the maximum cross section is increased and shifted to higher energies such that the calculated spectra come to lie above the experimental ones. From this an adjustment of the cross sections try t o , seemed to be meaningful for E~ = 50-450 MeV. For this the compound nucleus formation cross section was no longer taken to be the total ",,-absorption cross section, but was used as a free parameter in a search. For these fits the ),-energy interval was divided into two parts corresponding to the two absorption mechanisms assumed to be effective: the quasideuteron, and pion production and reabsorption processes. In fig. 7 the full lines represent the fits and table 4 gives the best fit parameters. From these values the part of the total ),-absorption cross section leading to compound nucleus formation in this energy interval can be


TABLE 4 The integrated absorption cross sections and the percentage values of compound nucleus formation

dE;, (MeV) 50-150 150-250 250-350 350-450 J'0-./t.tdET, 0-c7/0-7 tot ~0-~ dE .) (%) 7 (b" MeV)

27A1 tr 7 to t 6 10 15 12 4.3 a" 7 tot -" a¢-/ 6.2 7.6 2.9 69

4°Ca cry tot 8 15 22 17 6.2 0-¢7 7.2 I0.1 3.75 60

5IV O" 7 to t 1 1 19 27 22 7.9 0-c 7 13 0.01 1.3 16.5

93Nb 0-3' tot 19 35 50 39 15.3 0-¢./ 5.8 5.9 2.3 15

n~'Ag 0-;, to t 22 40 58 47 16.7 0-~7 15 7.6 3.81 23

The 0-~, to t values (in mb) used in the evaporation calculation in the ~,-energy range E 7 = 50-450 MeV and corresponding best fit values a'7 ,or are given. The formation cross section of the compound nucleus trc,, is assumed to be equal to tr 7 to t or O'~ t o t ,

;) E 7 ='50-450 MeV.

estimated. The integrated absorption cross sections and the percentage values of compound nucleus formation are also given in table 4. While the total absorption cross section increases with A, the formation cross section turns out to be approximately constant, which is reflected in the decrease of the percentage values.

5.2. THE 0~-PARTICLE YIELDS

The or-particle yields reach a m a x i m u m at target mass numbers A ~ 50 and then

fall off to an app rox ima te ly cons tan t value o f a b o u t ha l f the m a x i m u m value.

Assuming c o m p o u n d nucleus fo rmat ion , the occurance o f a m a x i m u m m a y be

expla ined by the two factors con ta ined in tr~.~: trr~ = tr~FJFto t (Bohr ' s hypothes is ) ;

the in tegra ted y-absorp t ion cross section increases, whereas the b ranch ing ra t io for

the ~-channel decreases with A. The shape o f the exper imenta l yield curve is, there-

fore, well r ep roduced by the stat ist ical mode l calculat ions for the l ight nuclei,

giving the m a x i m u m and even the s t rong dip for 5tV and the high yield for 93Nb.

Both the la t te r effects are assumed to be consequences o f the relat ively high or low

value o f the intensi ty threshold for the (~, ct) process defined as the sum of the

separa t ion energy E B plus the nomina l C o u l o m b bar r ie r o f two touching hard ,

charged spheres. The values o f these intensi ty thresholds are given in table 5 for the

var ious target nuclei and react ions. F o r the (~, ~t) reac t ion they increase f rom 15.5 MeV for 4°Ca to 21.6 MeV for t97Au. The value for s lV is 19.4 MeV, which is to be

compared to the values of abou t 17 MeV in this mass region.

158 P. DAVID et al.

TABLE 5

Intensity thresholds (in MeV) for the reactions (?', n), (7, P), (~', t), (7, 3He) and (7, "*He)

Target (7, n) (7, P) (~', t) (~,, 3He) (~,, 4He)

27AI 13.5 12.63 22.69 29.44 15.88 *°Ca 15.61 14.54 31.30 27.17 15.47 '**Ca 11.13 18.17 26.48 31.45 17.04

5 i V 11.05 14.67 25.37 31.72 19.43 59Co 10.46 14.82 24.11 30.64 17.43 93Nb 8.82 15.84 23.27 29.82 16.18 aatAg 9.35 16.85 16.85 32.16 18.05

18 iTa 7.64 20.32 20.32 34.56 19.87 197Au 8.08 20.62 20.62 36.07 21.61

13::

300 e('~ct)E~. =8-20MeV o(V.3He) E~ =7-20MeV

" H e

250 ! t J.

2OO

3O I i I

50 100 150~LAB

Fig. 9. Angular distributions of 4°Ca(y, ct) and '*°Ca( 7, 3He) reactions at E~ "~ = 450 MeV.

For 93Nb the low intensity threshold of 16.2 MeV should give rise to an enhanced

s-particle yield. Also the relation of this threshold to the position of the maximum of the G D R

influences the yields. If the particle threshold is well above the G D R a smaller amount of 7-quanta will lead to particle emission and vice versa, when it is lower or of the same value.

The influence of the position of the intensity threshold relatively to the G D R maximum should clearly be reflected in the or-particle angular distributions.

For 4 ° C a the intensity threshold is 15.5 MeV whereas.the maximum of the G D R is located at 20 MeV [ref. x 7)]. If the s-yield mainly results from *~-dipole-absorption processes the angular distribution should have a maximum at 90 °. This is seen in fig. 9. A forward peaked angular distribution was measured by Adler et al. 8) for

Au(y, ~), at E~ a" = 1 GeV.


The :c-particle spectrum from 4°Ca shows a higher intensity at low energies than the spectrum from 44Ca.

The ~t-particle spectra of the heavy targets seem to be more complex. For nuclei XSlTa and 197Au the calculations reach the experimental cross sections, but the whole spectra are displayed to higher energies. Also efforts to fit the form of the spectra using only the Maxwellian distribution of the evaporation model fail in these cases. The large number of low energy a-particles might be explained by other processes, e.g., ,~-particles emitted in fission processes. Fraenkel is) estimated the cross section for s-particles from fission fragments to be roughly ~ 0 of the fission cross section trq itself. For 197Au the fission cross section is aq = 2 mb [ref. 2)] at E~ ax = 450 MeV. The cross section for the s-particles emitted from fission fragments is therefore of the same order of magnitude as the cross section of the photon induced s-particle reaction. On the other hand it is well known that s-spectra produced from heavy targets by particles of about 60 MeV [refs. x9' 2o)] are not well reproduced by the statistical model.

5.3. THE (~', p) REACTION

The experimental proton energy spectra (figs. 3c) show higher cross sections but steeper fall off, if compared to the or-spectra. The maxima of the distributions were not measured except for 197Au. The calculations do not reproduce these spectra, giving much lower cross sections and a wrong position of the maxima. Fig. 6 shows the experimental data and the calculations for the nuclei 4°Ca and 27AI. As seen in the calculated cross sections for c~-emission, the intensity threshold strongly governs the result. The underestimation of the proton yields in these calculations is therefore explained by the influence of the (7, P) thresholds. They are only little lower compared to the (7, ~) values, giving rise to about the same yields in either reaction.

The experimentally observed large number of protons is therefore attributed to direct processes in the target nucleus, which is left in a highly excited state. This is supported by (7, ~) measurements, where the maximum bremsstrahlung energy is of the order of the energy where the GDR has its maximum 11). These spectra show maxima in the p-energy range covered in the measurements of this work and much lower yields. This comparison with spectra from proton induced reactions for Ep = 29 and 62 MeV [ref. t 9)], too, shows the strong contribution of low energetic protons especially for heavy nuclei. The absolute cross sections are much lower for v-induced reactions because of the lower v-absorption cross section.

The proton yield as a function of target mass A is larger than the or-particle yield, but the A-dependence is nearly identical. The integrated v-absorption cross section increasing with A and the decreasing branching ratio Fp/Fto t determine the yield curve and give the maximum at A g 50. For heavy targets the increasing yield of protons from other than compound processes again enhances the yield.

160 P. DAVID et al.

5.4. THE (y, t) AND THE (y, aHe) REACTIONS

The statistical errors of the triton and 3He particle spectra displayed representatively in figs. 3a, b and 3d, are large. For this reason no calculations were performed in the statistical model; this was also because neither branching ratios for these channels nor capture cross sections are known.

The triton spectra can be compared with results of measurements in the giant dipole resonance region 11) and with the results of an experiment at E~ ax = 90 MeV [ref. 22)]. Only the spectra of the light and medium heavy nuclei can be explained in the frame- work of the statistical model; this result was found for the ~-particle spectra already. The triton yields are of the same order of magnitude as the ~-particle yields or somewhat lower, although the intensity thresholds are about 6 MeV higher (table 5). An exception is 4°Ca, with a threshold of 31.3 MeV, which may explain the low yield from this nucleus, this even being smaller than the yield from 27A1.

The relatively low triton yield from 197Au may indicate that emission of tritons from fission processes or products is small compared with the ~-particles.

The 3I-[e spectra reflect the smaller yield compared with the other particles. Only the spectra from the light target nuclei have an indication of a maximum; the distributions from the heavy nuclei, eg. t StTa and t97Au, show no structure at all. The low yields may be explained again by the high intensity threshold, which for aHe particles ranges from 27.2 MeV for 4°Ca to 36.1 MeV for 197Au (table 5). Because of this high intensity threshold, the emission of 3He particles mainly results from absorption processes of ?-rays with energies higher than the threshold leading to highly excited nuclei. This is supported by the angular distribution of aHe particles from 4°Ca, displayed in fig. 9. It is seen to be isotropic, excluding contributions from the GDR region. Up to now no 3He spectra induced by y-rays have been published in the literature. The 3He spectra from proton induced reactions at 62 MeV on heavy nuclei 19) are found to be very similar in shape and absolute cross section. Comparing the 3He spectra from the proton induced reaction at 90 ° it is seen that the absolute cross sections coincide with those of the s-spectra at energies for which the statistical model seems to be no longer applicable. From this one may conclude that other processes are responsible for the emission of 3He particles.

The yield for aHe particles as a function of A shows the same behaviour as the other particle yields do, with a maximum at A ~ 50. The irregularity for S~V can be explained by the high intensity threshold.

We are grateful for the permanent support and the stimulating interest of Prof. Dr. T. Mayer-Kuckuk in these investigations and acknowledge the cxcellent working conditions in his Institute. Prof. Dr. K. H. Althoff, Prof. Dr. G. Knop, Prof. Dr. G. N61deke and Prof. Dr. W. Paul made possible the measurements at the Bonn 0.5 GeV electron synchrotron. We gratefully acknowledge this and the encouraging discussions. We thank Dr. H. Genzel, A. Christ, Dr. B. Mecking, T. Reichelt and H. J. Weyer for their collaboration. We thank K. Eberhard for many stimulating


discussions and for providing the Ca targets. The excellent help of the synchrotron staff, J. Karthaus, P. Haas and K. Kiiffner is thankfully appreciated. This work was partly supported by the Landesamt far Forschung des Landes Nordrhein- Westfalen.

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Bremsstrahlung induced nuclear reactions with Eγmax = 450 MeV